To be (stable) or not to be
Birds regularly perform astounding aerial maneuvers, often believed to be possible because of their reduced stability. Instead, we found evidence that evolutionary pressures act to maintain birds capacity to shift between stable and unstable flight.
Modern aircraft can generally fly much faster and transport more cargo than birds, but aircraft still underperform birds in a key aspect of flight: maneuverability. Some of the maneuvers that birds perform during everyday flight – like making sudden dives or maintaining their position on gusty days – can surpass what is possible in even some of the most advanced aircraft. Over the course of their evolutionary history, birds have diversified into over 10,000 species, the majority of which are capable of flight. Flighted birds weigh between 20 and 0.002 kg, and with this four-order of magnitude body mass range comes broad diversity in physical features (Fig. 1). Though evolutionary forces do not necessarily produce “optimal” designs, the vast array of shapes, sizes, and flight control strategies shown by birds at least provides compelling starting points to inspire the design of more maneuverable aircraft.
Studies have pointed towards birds' ability to adjust the shape of their wings during flight (termed “wing morphing”, Fig. 2) as one of the major reasons that they are so maneuverable. Wings themselves come in many shapes and sizes: long, short, curved, flat, pointed, rounded, and nearly everything in between. The ability to “morph” the wing during flight gives birds additional control over these wing shapes, and in turn could give them more ability to control maneuvers. Quantifying maneuverability, however, is not trivial – it is broad term that encompasses many flight attributes. Instead, it is often more convenient to measure stability, which is easier to quantify and often has a trade-off with maneuverability.
What does it mean to be aerodynamically stable? Imagine a flying bird that is simply cruising forward and then it suddenly encounters a gust. The bird is considered stable if it inherently returns towards its original balanced position. If instead the gust causes the bird to tilt to an even more extreme position, the bird is unstable. In a nutshell, the bird is stable if, due to the bird’s shape and posture, there are some restorative forces and/or moments acting on the bird to return it to its balanced position. The intent of a maneuver, however, is the opposite: to leave the balanced position. The stronger the restorative forces acting on the bird, the higher the maneuvering (control) forces must be to perform the maneuver. A highly stable bird would require more active control (e.g., more effort to change to how it is positioning its wings or body) to maneuver effectively. Since modern birds perform impressive maneuvers, it’s been commonly believed that they are unstable, but this has remained largely unverified.
To quantify the stability of birds, we turned to traditional aircraft methods. The methods we used principally relied on understanding two properties of the aircraft or animal in flight: 1) the center of gravity and 2) the neutral point. The center of gravity is the location within the bird about which all its mass is balanced. The neutral point is the location where, if the center of gravity were placed, the bird would be neutrally stable. If the neutral point is located behind the bird’s center of gravity, known as a “positive static margin”, the bird is aerodynamically stable (Fig. 3a). This is analogous to a ball being nudged while it is inside of a concave surface, like a bowl (Fig. 3b). If, instead, the neutral point is in front of the center of gravity (negative static margin), the bird would be unstable (Fig. 3c). This is analogous to a ball being nudged while it is on a convex surface (Fig. 3d).
We calculated these metrics, among others, by taking morphological measurements from 22 species of birds hailing from many of the prominent lineages of birds. We flexed and extended the wings of these species through their range of motion to track how wing posture affects the shape of the wing. To calculate how the distribution of mass within a bird might then be affected by changes in wing shape, neck posture and/or tail posture, we developed a new open-source software (AvInertia). Collectively, our data and software allowed us to understand how changes to wing shape and posture would affect the center of gravity, the neutral point, and in turn, the maximum and minimum values of the static margin.
Our findings change our fundamental understanding of stability in bird flight. Strikingly, the majority of the investigated birds have the capacity to shift between stable and unstable flight by adjusting wing shape and posture (Fig. 4). Of the 22 species in our study, 1 species was entirely unstable, 4 were entirely stable, and 17 could shift between both stable and unstable flight. Via mathematical modeling, we also found evidence that evolutionary pressures collectively appear to, across birds, maintain a maximum static margin that is stable (max dashed line, Fig. 4) and also a minimum static margin that is unstable (min dashed line, Fig. 4). These findings contrast with the existing notion that birds are evolving towards being only unstable.
Bird flight control systems are clearly more complex than previously assumed. In aircraft, operating and transitioning smoothly between stable and unstable flight requires significantly different control algorithms. This, in turn, underscores the importance of ongoing work to better understand the neurological control of bird flight. Determining birds’ physical capacity to shift between stable and unstable conditions is, however, an important first step towards providing biologically-informed designs for more maneuverable aircraft.